1. Field of the Invention
This invention relates to an oxygen concentration detector for detecting oxygen concentration of a gaseous mixture with an oxygen concentration cell using an oxygen ion conductive solid electrolyte body.
2. Description of the Prior Art
To measure the oxygen concentration of the exhaust gas from an internal combustion engine or the like, an oxygen concentration detector having an oxygen ion conductive solid electrolyte body and based on the principle of an oxygen concentration cell has been known. The oxygen concentration detector of the prior art generally comprises a solid electrolyte body made of, for example, yttria added zirconia ceramic and electrodes made of platinum. Such an oxygen concentration detector has been usually used for controlling air-fuel ratio .lambda. to maintain it at 1.0. However, it is often necessary to control the air-fuel ratio .lambda. to values other than 1.0 in order to use the engine in a lean burn zone including much air for higher fuel efficiency or a rich burn zone including less air for higher output efficiency. For this purpose, it is also necessary to precisely measure the electromotive force of the oxygen concentration cell and temperature.
Various oxygen concentration detectors simultaneously measuring the temperature and electromotive force have been proposed. For example, as shown in FIG. 1, a solid electrolyte body 1 in the form of a bottomed cylinder having electrodes 5 and 4 on its inner and outer surfaces includes a temperature sensor element 2 therein for measuring the temperature of the solid electrolyte body 1 or a coiled heating wire 3 therein for heating the solid electrolyte body 1 or includes simultaneously a temperature sensor element 2 and coiled heating wire 3 for heating the body and detecting the temperature thereof. These detectors could not precisely measure the temperature of the solid electrolyte body because they detect temperatures only at one point notwithstanding the temperature of the solid electrolyte body exposed to exhaust gases of an internal combustion engine is not uniform. Moreover, when the temperature of the exhaust gas varies, the temperature sensor element could not immediately respond to the change in temperature of the solid electrolyte body resulting in delayed output of the temperature detector element. In addition, with these oxygen concentration detectors, at low temperatures the catalytic action of the platinum become lower and the electric resistance of the solid electrolyte body itself becomes higher in response to increase of impedance of the detectors, so that they are susceptible to noise and tend to delay in response, with the result that they are in practise useless in conjunction with their complicated construction.
Furthermore, it has been proposed to vary the air-fuel ratio to values other than 1.0 causing a change in electromotive force by flowing a direct current in an oxygen concentration cell. However, this method could not precisely detect the temperature because of its great intensity of polarization at low temperatures and of great variation in excess-air ratio .lambda. depending upon temperatures.